Widely spotlighted as a semiconductor light emitting device, LEDs (Light Emitting Diodes) are applied to backlight light sources, display light sources, general light sources, full-color displays, etc., by virtue of properties of compound semiconductors. Typical LED materials are known to be Group III-V nitride semiconductors such as GaN (Gallium Nitride), AlN (Aluminum Nitride), InN (Indium Nitride), etc., and have direct transition type large energy band gaps and thus possess properties adapted for photoelectronic devices, including an ability to obtain almost the entire wavelength of light depending on the composition of nitride. Light emitting devices using such materials are applied in a variety of fields including flat panel displays, optical communication, etc.
Such devices are manufactured in the form of a thin film on a substrate using a growth process such as molecular beam epitaxy (MBE), MOCVD (MetalOrganic Chemical Vapor Deposition), HVPE (Hydride Vapor Phase Epitaxy), etc.
However, semiconductors based on Group III nitrides typically exemplified by GaN are configured such that a device structure is manufactured on a (0001) plane using a c-plane substrate (e.g. a sapphire substrate). In this case, spontaneous polarization is formed in the growth orientation (0001). Particularly in the case of LED having a quantum well structure of InGaN/GaN, when a structure is grown on the (0001) plane, internal strain is caused by lattice mismatch in the quantum well structure, and thus a quantum-confined Stark effect (QCSE) is created due to piezoelectric fields. Hence, limitations are imposed on increasing internal quantum efficiency.
Specifically, Group III nitrides, especially GaN and its alloys (e.g. alloys with InN and/or AlN), are the most stable in a hexagonal wurtzite structure, wherein the crystal structure is configured such that crystals are rotated at 120° with respect to each other, and is represented by two or three equivalent basal plane axes perpendicular to the c-axis.
Any plane perpendicular to the c-axis contains only one type of atom due to the positions of the Group III element and the nitrogen atom in the wurtzite crystal structure. Respective planes may contain one type of atom (Group III element or nitrogen) toward the c-axis. As such, in order to maintain the neutral charge, for example, GaN crystals are configured such that an N-face containing only nitrogen atoms and a Ga-face containing only Ga atoms are positioned at the ends thereof. Consequently, Group III nitride crystals show polarity along the c-axis. Such spontaneous polarization is a bulk property and depends on the structure and composition of crystals. Because of the above properties, most of GaN-based devices are mainly grown in a direction parallel to the polar c-axis. Furthermore, when a heterojunction structure is formed, stress is generated due to a large difference in lattice constant between the Group III nitrides and the same c-axis orientation, and thus piezoelectric polarization is also caused.
In the c-plane quantum well structure in the Group III nitride-based photoelectronic and electronic devices, an electrostatic field caused by piezoelectric polarization and spontaneous polarization may change the energy band structure of the quantum well structure and thus electron-hole distribution may become distorted. The spatial separation of electrons and holes due to such an electric field refers to a quantum-confined Stark effect, which decreases internal quantum efficiency and causes red shift of the light emission spectrum.
To solve the above problems, for example, methods of growing a non-polar or semi-polar Group III nitride are being proposed. The resulting non-polar or semi-polar plane contains the same number of Group III atoms and nitrogen atoms and shows the neutral charge, and no crystals polarize in the growth orientation. However, non-polar Group III nitride crystals growing on the heterogeneous substrate show high defect density, undesirably lowering quantum efficiency.
Meanwhile, in order to achieve homoepitaxial properties, many attempts are being made to manufacture Group III nitride substrates, wherein a thick Group III nitride layer is grown on a heterogeneous substrate such as a sapphire substrate, and the grown Group III nitride layer is then separated from the heterogeneous substrate using a laser lift off (LLO) process and thus used as a substrate.
The reason why the LLO process is used is that a typical nitride layer grown in the c-axis orientation shows Ga-polarity at the surface thereof, making it difficult to actually perform wet etching. However, because the LLO process incurs high costs and is complicated, the use of a chemical lift off process is more preferable.
Furthermore, even when a Group III nitride layer for a non-polar or semi-polar substrate is formed, it is not easy to separate it using a chemical lift off process. This is because the Group III nitride layer useful as a substrate may be damaged in a chemical etching process. Moreover, this problem may be further exacerbated in the course of separating a substrate having a diameter of at least 2 inches adapted for commercialization.
Thus, there is a need for techniques for manufacturing a non-polar or semi-polar Group III nitride substrate, especially a semi-polar Group III nitride substrate, which may solve problems due to polarity with the use of a chemical lift off process.